This invention relates to a fabrication technology for a semiconductor device and, more particularly, to a process for fabricating a semiconductor device, a fabrication apparatus used therein and a method for entering a semiconductor wafer into the fabrication apparatus.
An integrated circuit has been complicated, and the manufacturer is required to integrate a large number circuit components on a small semiconductor chip. The circuit component such as a field effect transistor is miniaturized, and the miniature field effect transistor requires an extremely thin gate oxide layer. When the manufacturer designs a field effect transistor under 0.25 micron design rules, the gate oxide layer is of the order of 8 nanometers thick. Now, the manufacturers are targeting a gate oxide layer of 5 nanometers thick or less.
Typical examples of oxidation technology are written in xe2x80x9cApparatus Technologies for Realizing Gate Oxide Equal to or Less Than 5 nm (DSI)xe2x80x9d, monthly magazine xe2x80x9cSemiconductor Worldxe2x80x9d, July 1996, pages 103 to 108. Silicon wafers are inserted into a quartz boat, and the quartz boat is inserted into a furnace. The furnace chamber is purged with nitrogen, and residual oxygen at the entrance for the boat is of the order of 2 to 3 percent. However, natural oxide is grown to 10 to 20 angstroms thick on the silicon wafers in the furnace chamber at 800 degrees in centigrade. The nitrogen is smaller in molecular weight than the air, and hardly pushes out the air from the gaps between the silicon wafers. In other words, the air is hardly evacuated from the gaps between the silicon wafers.
The article proposes several apparatus against undesirable growth of natural oxide. One of the apparatus against the undesirable oxidation is a load-lock chamber provided under an entrance of a vertical oxidation diffusion furnace, and nitrogen gas flows across the gap between the load-lock chamber and the entrance of the furnace chamber. A lifter is installed in the load-lock chamber, and a boat is mounted on the lifter. The lifter is upwardly moved, and inserts the board into the furnace chamber. The nitrogen gas is expected to blow the air off.
Another apparatus is a nitrogen purge box attached to the furnace entrance. The boat is placed in the nitrogen purge box before the entry into the furnace chamber, and a large amount of nitrogen is blown into the nitrogen purge box. The nitrogen is expected to enter into the gaps between the silicon wafers, and the residual air is replaced with the nitrogen.
Yet another apparatus is a double wall furnace. The boat is inserted into the inner tube, and the nitrogen gas is blown into the inner tube. The air between the silicon wafers is replaced with the nitrogen. The inner tube is inserted into the outer tube, and the silicon wafer is heated.
The load-lock chamber and the nitrogen purge box are attached to a conventional furnace, and the manufacturer requires cost for remodeling the furnace. The double wall furnace is complicated, and is expensive. In any case, the manufacturer requires large cost for replacing the conventional furnace with the double wall furnace. Thus, the apparatus proposed in the article are not economical.
A prior art lateral oxidation diffusion furnace is proposed in Japanese Patent Publication of Unexamined Application No. 4-162526. The Japanese Patent Publication of Unexamined Application proposes to use inert gas larger in molecular weight than nitrogen for the inert gas purge. Argon is the inert gas larger in molecular weight than the nitrogen.
FIG. 1 illustrates a typical example of the vertical oxidation diffusion furnace, and the inert gas purge disclosed in the Japanese Patent Publication is applied to the vertical oxidation diffusion furnace. The prior art vertical oxidation diffusion furnace largely comprises a quartz tube 1, a lifter 2 and a quartz boat 3. The quartz tube 1 defines a furnace chamber 1a, and has an entrance 1b open to the atmosphere at the lower end thereof. The lifter 2 includes a table 2a and a drive mechanism 2b, and the table 2a is aligned with the entrance 1b. The driving mechanism 2b upwardly moves the table 2a and the vice versa, and the entrance 1b is closed by the table 2a. A warmer 4 is placed on the table 2a, and the quartz boat 3 is stacked on the warmer 4. A gas nozzle 5 and a gas exit 6 are provided to the quartz tube 1 around the entrance 1b. The gas nozzle 5 is connected to an argon gas source (not shown), and the gas exit 6 is connected to a vacuum source.
A manufacturer oxidizes silicon wafers 7 or diffuses dopant impurity into the silicon wafers 7 as follows. The driving mechanism 2b moves the table 2a to the lower limit, and the entrance 1b becomes open to the atmosphere. The silicon wafers 7 are inserted into the quartz boat 3, and the quartz boat 3 is placed on the warmer 4. The driving mechanism 2b upwardly moves the table 2a, and the quartz boat 3 is inserted into the furnace chamber 1a. The entrance 1b is closed. The argon gas is introduced from the gas nozzle 5 into the furnace chamber 1a, and is exhausted from the furnace chamber through the gas exit 6. Thus, the air is gradually evacuated from the furnace chamber 1a, and is replaced with the argon gas.
FIG. 2 illustrates another prior art vertical oxidation diffusion furnace. The difference from the prior art vertical oxidation diffusion furnace shown in FIG. 1 is the location of the gas nozzle 5, and components of the vertical oxidation diffusion furnace are labeled with the same references designating corresponding components of the prior art vertical oxidation diffusion furnace shown in FIG. 1.
The argon gas is larger in molecular weight than the air and the oxygen, and is expected to push out the residual oxygen from the gaps between the silicon wafers 7. However, a problem is encountered in the prior art furnaces in that the residual air is left in the furnace chamber, and the silicon wafers 7 are undesirably oxidized by the oxygen of the residual air.
It is therefore an important object of the present invention to provide a process for fabricating a semiconductor device, in which oxidation of a semiconductor wafer is precisely controlled without cost.
It is also an important object of the present invention to provide an apparatus for precisely controlling the oxidation of the semiconductor wafer.
It is also an important object of the present invention to provide a method for entering a semiconductor wafer into the apparatus.
The present inventor contemplated the problem inherent in the prior art, and noticed that the argon gas got under the residual air in the furnace chamber. The boat 3 is upwardly protected from the entrance 1b. Although the air was gradually replaced with the argon gas, the air was hardly evacuated from the upper portion of the furnace chamber 1a, and the residual air oxidized the silicon wafer at a higher level. The present inventor measured the residual oxygen in the furnace chamber 1a, and the oxygen concentration was greater than that in the prior art load-lock chamber, i.e., 10 ppm to 50 ppm. Thus, there was a trade-off between the inert gas lighter than the air and the inert gas heavier than the air. The present inventor concluded to share the evacuation work between different kinds of inert gas.
To accomplish the object, the present invention proposes to use gaseous mixture between a first kind of inert gas lighter than the oxygen and a second kind of inert gas heavier than the oxygen for the evacuation work.
In accordance with one aspect of the present invention, there is provided a process for fabricating a semiconductor device comprising the steps of preparing a semiconductor wafer having a portion of a first material reactive with oxygen, supplying a first purge gas less reactive with the first material and smaller in molecular weight than the oxygen and a second purge gas less reactive with the first material and larger in molecular weight than the oxygen during an insertion of the semiconductor wafer into a chamber, and carrying out a predetermined treatment on the portion in the chamber.
In accordance with another aspect of the present invention, there is provided an apparatus used in a fabrication of a semiconductor device comprising a wall defining a chamber having an entrance, a loader for inserting a semiconductor wafer having a portion of a first material reactive with oxygen through the entrance into the chamber, and a gas supplier connected to the wall and supplying a first purge gas less reactive with the first material and smaller in molecular weight than the oxygen and a second purge gas less reactive with the first material and larger in molecular weight than the oxygen into the chamber.
In accordance with yet another aspect of the present invention, there is provided a method for entering a semiconductor wafer into a chamber comprising the steps of preparing a semiconductor wafer having a portion of a certain material reactive with oxygen, supplying a first purge gas less reactive with the certain material and smaller in molecular weight than the oxygen and a second purge gas less reactive with the certain material and larger in molecular weight than the oxygen into a chamber, and inserting the semiconductor wafer into the chamber.